Kỹ thuật lập trình_Module 1

Kỹ thuật lập trình_Module 1

If there is one language that defines the essence of programming today, it is C++. It is the preeminent language for the development of high-performance software. Its syntax has become the standard for professional programming languages, and its design philosophy reverberates throughout computing

C++ is also the language from which both Java and C# are derived. Simply stated, to be a professional
programmer implies competency in C++. It is the gateway to all of modern programming.
The purpose of this module is to introduce C++, including its history, its design philosophy, and several
of its most important features. By far, the hardest thing about learning a programming language is the
fact that no element exists in isolation. Instead, the components of the language work together. This
interrelatedness makes it difficult to discuss one aspect of C++ without involving others. To help
overcome this problem, this module provides a brief overview of several C++ features, including the
general form of a C++ program, some basic control statements, and operators. It does not go into too
many details, but rather concentrates on the general concepts common to any C++ program.
CRITICAL SKILL 1.1: A Brief History of C++
The history of C++ begins with C. The reason for this is easy to understand: C++ is built upon the
foundation of C. Thus, C++ is a superset of C. C++ expanded and enhanced the C language to support
object-oriented programming (which is described later in this module). C++ also added several other
improvements to the C language, including an extended set of library routines. However, much of the
spirit and flavor of C++ is directly inherited from C. Therefore, to fully understand and appreciate C++,
you need to understand the “how and why” behind C.
C: The Beginning of the Modern Age of Programming
The invention of C defines the beginning of the modern age of programming. Its impact should not be
underestimated because it fundamentally changed the way programming was approached and thought
about. Its design philosophy and syntax have influenced every major language since. C was one of the
major, revolutionary forces in computing.
C was invented and first implemented by Dennis Ritchie on a DEC PDP-11 using the UNIX operating
system. C is the result of a development process that started with an older language called BCPL. BCPL
was developed by Martin Richards. BCPL influenced a language called B, which was invented by Ken
Thompson and which led to the development of C in the 1970s.
Prior to the invention of C, computer languages were generally designed either as academic exercises or
by bureaucratic committees. C was different. It was designed, implemented, and developed by real,
working programmers, reflecting the way they approached the job of programming. Its features were
honed, tested, thought about, and rethought by the people who actually used the language. As a result,
C attracted many proponents and quickly became the language of choice of programmers around the
world.
C grew out of the structured programming revolution of the 1960s. Prior to structured programming,
large programs were difficult to write because the program logic tended to degenerate into what is
known as “spaghetti code,” a tangled mass of jumps, calls, and returns that is difficult to follow.
Structured languages addressed this problem by adding well-defined control statements, subroutines
2 C++ A Beginner’s Guide by Herbert Schildt

with local variables, and other improvements. Using structured languages, it became possible to write
moderately large programs.
Although there were other structured languages at the time, such as Pascal, C was the first to
successfully combine power, elegance, and expressiveness. Its terse, yet easy-to-use syntax coupled
with its philosophy that the programmer (not the language) was in charge quickly won many converts. It
can be a bit hard to understand from today’s perspective, but C was a breath of fresh air that
programmers had long awaited. As a result, C became the most widely used structured programming
language of the 1980s.
The Need for C++
Given the preceding discussion, you might be wondering why C++ was invented. Since C was a successful
computer programming language, why was there a need for something else? The answer is complexity.
Throughout the history of programming, the increasing complexity of programs has driven the need for
better ways to manage that complexity. C++ is a response to that need. To better understand the
correlation between increasing program complexity and computer language development, consider the
following.
Approaches to programming have changed dramatically since the invention of the computer. For
example, when computers were first invented, programming was done by using the computer’s front
panel to toggle in the binary machine instructions. As long as programs were just a few hundred
instructions long, this approach worked. As programs grew, assembly language was invented so that
programmers could deal with larger, increasingly complex programs by using symbolic representations
of the machine instructions. As programs continued to grow, high-level languages were developed to
give programmers more tools with which to handle the complexity.
The first widely used computer language was, of course, FORTRAN. While FORTRAN was a very
impressive first step, it is hardly a language that encourages clear, easy-to-understand programs. The
1960s gave birth to structured programming, which is the method of programming encouraged by
languages such as C. With structured languages it was, for the first time, possible to write moderately
complex programs fairly easily. However, even with structured programming methods, once a project
reaches a certain size, its complexity exceeds what a programmer can manage. By the late 1970s, many
projects were near or at this point.
In response to this problem, a new way to program began to emerge: object-oriented programming
(OOP). Using OOP, a programmer could handle larger, more complex programs. The trouble was that C
did not support object-oriented programming. The desire for an object-oriented version of C ultimately
led to the creation of C++.
In the final analysis, although C is one of the most liked and widely used professional programming
languages in the world, there comes a time when its ability to handle complexity is exceeded. Once a
program reaches a certain size, it becomes so complex that it is difficult to grasp as a totality. The
3 C++ A Beginner’s Guide by Herbert Schildt

purpose of C++ is to allow this barrier to be broken and to help the programmer comprehend and
manage larger, more complex programs.
C++ Is Born
C++ was invented by Bjarne Stroustrup in 1979, at Bell Laboratories in Murray Hill, New Jersey. He
initially called the new language “C with Classes.” However, in 1983 the name was changed to C++.
Stroustrup built C++ on the foundation of C, including all of C’s features, attributes, and benefits. He also
adhered to C’s underlying philosophy that the programmer, not the language, is in charge. At this point,
it is critical to understand that Stroustrup did not create an entirely new programming language.
Instead, he enhanced an already highly successful language.
Most of the features that Stroustrup added to C were designed to support object-oriented
programming. In essence, C++ is the object-oriented version of C. By building upon the foundation of C,
Stroustrup provided a smooth migration path to OOP. Instead of having to learn an entirely new
language, a C programmer needed to learn only a few new features before reaping the benefits of the
object-oriented methodology.
When creating C++, Stroustrup knew that it was important to maintain the original spirit of C, including
its efficiency, flexibility, and philosophy, while at the same time adding support for object-oriented
programming. Happily, his goal was accomplished. C++ still provides the programmer with the freedom
and control of C, coupled with the power of objects.
Although C++ was initially designed to aid in the management of very large programs, it is in no way
limited to this use. In fact, the object-oriented attributes of C++ can be effectively applied to virtually
any programming task. It is not uncommon to see C++ used for projects such as editors, databases,
personal file systems, networking utilities, and communication programs. Because C++ shares C’s
efficiency, much high-performance systems software is constructed using C++. Also, C++ is frequently
the language of choice for Windows programming.
The Evolution of C++
Since C++ was first invented, it has undergone three major revisions, with each revision adding to and
altering the language. The first revision was in 1985 and the second in 1990. The third occurred during
the C++ standardization process. Several years ago, work began on a standard for C++. Toward that end,
a joint ANSI (American National Standards Institute) and ISO (International Standards Organization)
standardization committee was formed. The first draft of the proposed standard was created on January
25, 1994. In that draft, the ANSI/ISO C++ committee (of which I was a member) kept the features first
defined by Stroustrup and added some new ones. But, in general, this initial draft reflected the state of
C++ at the time.
Soon after the completion of the first draft of the C++ standard, an event occurred that caused the
standard to be greatly expanded: the creation of the Standard Template Library (STL) by Alexander
Stepanov. The STL is a set of generic routines that you can use to manipulate data. It is both powerful
4 C++ A Beginner’s Guide by Herbert Schildt

and elegant. But it is also quite large. Subsequent to the first draft, the committee voted to include the
STL in the specification for C++. The addition of the STL expanded the scope of C++ well beyond its
original definition. While important, the inclusion of the STL, among other things, slowed the
standardization of C++.
It is fair to say that the standardization of C++ took far longer than anyone had expected. In the process,
many new features were added to the language, and many small changes were made. In fact, the
version of C++ defined by the ANSI/ISO C++ committee is much larger and more complex than
Stroustrup’s original design. The final draft was passed out of committee on November 14, 1997, and an
ANSI/ISO standard for C++ became a reality in 1998. This is the specification for C++ that is usually
referred to as Standard C++.
The material in this book describes Standard C++. This is the version of C++ supported by all mainstream
C++ compilers, including Microsoft’s Visual C++. Thus, the code and information in this book are fully
portable.
CRITICAL SKILL 1.2: How C++ Relates to Java and C#
In addition to C++, there are two other important, modern programming languages: Java and C#. Java
was developed by Sun Microsystems, and C# was created by Microsoft. Because there is sometimes
confusion about how these two languages relate to C++, a brief discussion of their relationship is in
order.
C++ is the parent for both Java and C#. Although both Java and C# added, removed, and modified
various features, in total the syntax for these three languages is nearly identical. Furthermore, the
object model used by C++ is similar to the ones used by Java and C#. Finally, the overall “look and feel”
of these languages is very similar. This means that once you know C++, you can easily learn Java or C#.
The opposite is also true. If you know Java or C#, learning C++ is easy. This is one reason that Java and C#
share C++’s syntax and object model; it facilitated their rapid adoption by legions of experienced C++
programmers.
The main difference between C++, Java, and C# is the type of computing environment for which each is
designed. C++ was created to produce high-performance programs for a specific type of CPU and
operating system. For example, if you want to write a program that runs on an Intel Pentium under the
Windows operating system, then C++ is the best language to use.
Ask the Expert
Q: How do Java and C# create cross-platform, portable programs, and why can’t C++ do the same?
A: Java and C# can create cross-platform, portable programs and C++ can’t because of the type of
object code produced by the compiler. In the case of C++, the output from the compiler is machine code
5 C++ A Beginner’s Guide by Herbert Schildt

that is directly executed by the CPU. Thus, it is tied to a specific CPU and operating system. If you want
to run a C++ program on a different system, you need to recompile it into machine code specifically
targeted for that environment. To create a C++ program that would run in a variety of environments,
several different executable versions of the program are needed.
Java and C# achieve portability by compiling a program into a pseudocode, intermediate language. In
the case of Java, this intermediate language is called bytecode. For C#, it is called Microsoft Intermediate
Language (MSIL). In both cases, this pseudocode is executed by a runtime system. For Java, this runtime
system is called the Java Virtual Machine (JVM). For C#, it is the Common Language Runtime (CLR).
Therefore, a Java program can run in any environment for which a JVM is available, and a C# program
can run in any environment in which the CLR is implemented.
Since the Java and C# runtime systems stand between a program and the CPU, Java and C# programs
incur an overhead that is not present in the execution of a C++ program. This is why C++ programs
usually run faster than the equivalent programs written in Java or C#.
Java and C# were developed in response to the unique programming needs of the online environment of
the Internet. (C# was also designed to simplify the creation of software components.) The Internet is
connected to many different types of CPUs and operating systems. Thus, the ability to produce cross-
platform, portable programs became an overriding concern.
The first language to address this need was Java. Using Java, it is possible to write a program that runs in
a wide variety of environments. Thus, a Java program can move about freely on the Internet. However,
the price you pay for portability is efficiency, and Java programs execute more slowly than do C++
programs. The same is true for C#. In the final analysis, if you want to create high-performance software,
use C++. If you need to create highly portable software, use Java or C#.
One final point: Remember that C++, Java, and C# are designed to solve different sets of problems. It is
not an issue of which language is best in and of itself. Rather, it is a question of which language is right
for the job at hand.
1. From what language is C++ derived?
2. What was the main factor that drove the creation of C++?
3. C++ is the parent of Java and C#. True or False?
6 C++ A Beginner’s Guide by Herbert Schildt

Answer Key:
1. C++ is derived from C.
2. Increasing program complexity was the main factor that drove the creation of C++.
3. True.
CRITICAL SKILL 1.3: Object-Oriented Programming
Central to C++ is object-oriented programming (OOP). As just explained, OOP was the impetus for the
creation of C++. Because of this, it is useful to understand OOP’s basic principles before you write even a
simple C++ program.
Object-oriented programming took the best ideas of structured programming and combined them with
several new concepts. The result was a different and better way of organizing a program. In the most
general sense, a program can be organized in one of two ways: around its code (what is happening) or
around its data (who is being affected). Using only structured programming techniques, programs are
typically organized around code. This approach can be thought of as “code acting on data.”
Object-oriented programs work the other way around. They are organized around data, with the key
principle being “data controlling access to code.” In an object-oriented language, you define the data
and the routines that are permitted to act on that data. Thus, a data type defines precisely what sort of
operations can be applied to that data.
To support the principles of object-oriented programming, all OOP languages, including C++, have three
traits in common: encapsulation, polymorphism, and inheritance. Let’s examine each.
Encapsulation
Encapsulation is a programming mechanism that binds together code and the data it manipulates, and
that keeps both safe from outside interference and misuse. In an object-oriented language, code and
data can be bound together in such a way that a self-contained black box is created. Within the box are
all necessary data and code. When code and data are linked together in this fashion, an object is
created. In other words, an object is the device that supports encapsulation.
Ask the Expert
Q: I have heard the term method applied to a subroutine. Is a method the same as a function?
A: In general, the answer is yes. The term method was popularized by Java. What a C++ programmer
calls a function, a Java programmer calls a method. C# programmers also use the term method. Because
it is becoming so widely used, sometimes the term method is also used when referring to a C++
7 C++ A Beginner’s Guide by Herbert Schildt

function.
Within an object, code or data or both may be private to that object or public. Private code or data is
known to and accessible by only another part of the object. That is, private code or data cannot be
accessed by a piece of the program that exists outside the object. When code or data is public, other
parts of your program can access it even though it is defined within an object. Typically, the public parts
of an object are used to provide a controlled interface to the private elements of the object.
C++’s basic unit of encapsulation is the class. A class defines the form of an object. It specifies both the
data and the code that will operate on that data. C++ uses a class specification to construct objects.
Objects are instances of a class. Thus, a class is essentially a set of plans that specifies how to build an
object.
The code and data that constitute a class are called members of the class. Specifically, member
variables, also called instance variables, are the data defined by the class. Member functions are the
code that operates on that data. Function is C++’s term for a subroutine.
Polymorphism
Polymorphism (from Greek, meaning “many forms”) is the quality that allows one interface to access a
general class of actions. A simple example of polymorphism is found in the steering wheel of an
automobile. The steering wheel (the interface) is the same no matter what type of actual steering
mechanism is used. That is, the steering wheel works the same whether your car has manual steering,
power steering, or rack-and-pinion steering. Thus, turning the steering wheel left causes the car to go
left no matter what type of steering is used. The benefit of the uniform interface is, of course, that once
you know how to operate the steering wheel, you can drive any type of car.
The same principle can also apply to programming. For example, consider a stack (which is a first-in, last-
out list). You might have a program that requires three different types of stacks. One stack is used for
integer values, one for floating-point values, and one for characters. In this case, the algorithm that
implements each stack is the same, even though the data being stored differs. In a non–object-oriented
language, you would be required to create three different sets of stack routines, with each set using
different names. However, because of polymorphism, in C++ you can create one general set of stack
routines that works for all three situations. This way, once you know how to use one stack, you can use
them all.
More generally, the concept of polymorphism is often expressed by the phrase “one interface, multiple
methods.” This means that it is possible to design a generic interface to a group of related activities.
Polymorphism helps reduce complexity by allowing the same interface to specify a general class of
action. It is the compiler’s job to select the specific action (that is, method) as it applies to each
situation. You, the programmer, don’t need to do this selection manually. You need only remember and
utilize the general interface.
8 C++ A Beginner’s Guide by Herbert Schildt

Inheritance
Inheritance is the process by which one object can acquire the properties of another object. This is
important because it supports the concept of hierarchical classification. If you think about it, most
knowledge is made manageable by hierarchical (that is, top-down) classifications. For example, a Red
Delicious apple is part of the classification apple, which in turn is part of the fruit class, which is under
the larger class food. That is, the food class possesses certain qualities (edible, nutritious, and so on)
which also, logically, apply to its subclass, fruit. In addition to these qualities, the fruit class has specific
characteristics (juicy, sweet, and so on) that distinguish it from other food. The apple class defines those
qualities specific to an apple (grows on trees, not tropical, and so on). A Red Delicious apple would, in
turn, inherit all the qualities of all preceding classes and would define only those qualities that make it
unique.
Without the use of hierarchies, each object would have to explicitly define all of its characteristics. Using
inheritance, an object need only define those qualities that make it unique within its class. It can inherit
its general attributes from its parent. Thus, it is the inheritance mechanism that makes it possible for
one object to be a specific instance of a more general case.
1. Name the principles of OOP.
2. What is the basic unit of encapsulation in C++?
3. What is the commonly used term for a subroutine in C++?
Answer Key:
1. Encapsulation, polymorphism, and inheritance are the principles of OOP.
2. The class is the basic unit of encapsulation in C++.
3. Function is the commonly used term for a subroutine in C++.
Ask the Expert
Q: You state that object-oriented programming (OOP) is an effective way to manage large programs.
However, it seems that OOP might add substantial overhead to relatively small ones. As it relates to C++,
9 C++ A Beginner’s Guide by Herbert Schildt

is this true?
A: No. A key point to understand about C++ is that it allows you to write object-oriented programs, but
does not require you to do so. This is one of the important differences between C++ and Java/C#, which
employ a strict object-model in which every program is, to at least a small extent, object oriented. C++
gives you the option. Furthermore, for the most part, the object-oriented features of C++ are
transparent at runtime, so little (if any) overhead is incurred.
CRITICAL SKILL 1.4: A First Simple Program
Now it is time to begin programming. Let’s start by compiling and running the short sample C++ program
shown here:
/*
This is a simple C++ program.
Call this file Sample.cpp.
*/
#include
using namespace std;
// A C++ program begins at main().
int main()
{
cout

Entering the Program
The programs shown in this book are available from Osborne’s web site: www.osborne.com. However, if
you want to enter the programs by hand, you are free to do so. Typing in the programs yourself often
helps you remember the key concepts. If you choose to enter a program by hand, you must use a text
editor, not a word processor. Word processors typically store format information along with text. This
format information will confuse the C++ compiler. If you are using a Windows platform, then you can
use WordPad, or any other programming editor that you like.
The name of the file that holds the source code for the program is technically arbitrary. However, C++
programs are normally contained in files that use the file extension .cpp. Thus, you can call a C++
program file by any name, but it should use the .cpp extension. For this first example, name the source
file Sample.cpp so that you can follow along. For most of the other programs in this book, simply use a
name of your own choosing.
Compiling the Program
How you will compile Sample.cpp depends upon your compiler and what options you are using.
Furthermore, many compilers, such as Microsoft’s Visual C++ Express Edition which you can download
for free, provide two different ways for compiling a program: the command-line compiler and the
Integrated Development Environment (IDE). Thus, it is not possible to give generalized instructions for
compiling a C++ program. You must consult your compiler’s instructions.
The preceding paragraph notwithstanding, if you are using Visual C++, then the easiest way to compile
and run the programs in this book is to use the command-line compilers offered by these environments.
For example, to compile Sample.cpp using Visual C++, you will use this command line:
C:\...>cl -GX Sample.cpp
The -GX option enhances compilation. To use the Visual C++ command-line compiler, you must first
execute the batch file VCVARS32.BAT, which is provided by Visual C++. (Visual Studio also provides a
ready-to-use command prompt environment that can be activated by selecting Visual Studio Command
Prompt from the list of tools shown under the Microsoft Visual Studio entry in the Start | Programs
menu of the taskbar.)
The output from a C++ compiler is executable object code. For a Windows environment, the executable
file will use the same name as the source file, but have the .exe extension. Thus, the executable version
of Sample.cpp will be in Sample.exe.
Run the Program
After a C++ program has been compiled, it is ready to be run. Since the output from a C++ compiler is
executable object code, to run the program, simply enter its name at the command prompt. For
example, to run Sample.exe, use this command line:
C:\...>Sample
11 C++ A Beginner’s Guide by Herbert Schildt

When run, the program displays the following output:
C++ is power programming.
If you are using an Integrated Development Environment, then you can run a program by selecting Run
from a menu. Consult the instructions for your specific compiler. For the programs in this book, it is
usually easier to compile and run from the command line.
One last point: The programs in this book are console based, not window based. That is, they run in a
Command Prompt session. C++ is completely at home with Windows programming. Indeed, it is the
most commonly used language for Windows development. However, none of the programs in this book
use the Windows Graphic User Interface (GUI). The reason for this is easy to understand: Windows
programs are, by their nature, large and complex. The overhead required to create even a minimal
Windows skeletal program is 50 to 70 lines of code. To write Windows programs that demonstrate the
features of C++ would require hundreds of lines of code each. In contrast, console-based programs are
much shorter and are the type of programs normally used to teach programming. Once you have
mastered C++, you will be able to apply your knowledge to Windows programming with no trouble.
The First Sample Program Line by Line
Although Sample.cpp is quite short, it includes several key features that are common to all C++
programs. Let’s closely examine each part of the program. The program begins with the lines
/*
This is a simple C++ program.
Call this file Sample.cpp.
*/
This is a comment. Like most other programming languages, C++ lets you enter a remark into a
program’s source code. The contents of a comment are ignored by the compiler. The purpose of a
comment is to describe or explain the operation of a program to anyone reading its source code. In the
case of this comment, it identifies the program. In more complex programs, you will use comments to
help explain what each feature of the program is for and how it goes about doing its work. In other
words, you can use comments to provide a “play-by-play” description of what your program does.
In C++, there are two types of comments. The one you’ve just seen is called a multiline comment. This
type of comment begins with a /* (a slash followed by an asterisk). It ends only when a */ is
encountered. Anything between these two comment symbols is completely ignored by the compiler.
Multiline comments may be one or more lines long. The second type of comment (single-line) is found a
little further on in the program and will be discussed shortly.
The next line of code looks like this:
#include
12 C++ A Beginner’s Guide by Herbert Schildt

The C++ language defines several headers, which contain information that is either necessary or useful
to your program. This program requires the header iostream, which supports the C++ I/O system. This
header is provided with your compiler. A header is included in your program using the #include
directive. Later in this book, you will learn more about headers and why they are important.
The next line in the program is
using namespace std;
This tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++.
Although namespaces are discussed in detail later in this book, here is a brief description. A namespace
creates a declarative region in which various program elements can be placed. Elements declared in one
namespace are separate from elements declared in another. Namespaces help in the organization of
large programs. The using statement informs the compiler that you want to use the std namespace. This
is the namespace in which the entire Standard C++ library is declared. By using the std namespace, you
simplify access to the standard library. (Since namespaces are relatively new, an older compiler may not
support them. If you are using an older compiler, see Appendix B, which describes an easy work-
around.)
The next line in the program is
// A C++ program begins at main().
This line shows you the second type of comment available in C++: the single-line comment. Single-line
comments begin with // and stop at the end of the line. Typically, C++ programmers use multiline
comments when writing larger, more detailed commentaries, and single-line comments when short
remarks are needed. This is, of course, a matter of personal style.
The next line, as the preceding comment indicates, is where program execution begins.
int main()
All C++ programs are composed of one or more functions. As explained earlier, a function is a
subroutine. Every C++ function must have a name, and the only function that any C++ program must
include is the one shown here, called main( ). The main( ) function is where program execution begins
and (most commonly) ends. (Technically speaking, a C++ program begins with a call to main( ) and, in
most cases, ends when main( ) returns.) The opening curly brace on the line that follows main( ) marks
the start of the main( ) function code. The int that precedes main( ) specifies the type of data returned
by main( ). As you will learn, C++ supports several built-in data types, and int is one of them. It stands for
integer.
The next line in the program is
cout

This is a console output statement. It causes the message C++ is power programming. to be displayed on
the screen. It accomplishes this by using the output operator

programmer, then decide whether the suspicion is justified.
Warnings are also used to report such things as inefficient constructs or the use of obsolete features.
Generally, you can select the specific type of warnings that you want to see. The programs in this book
are in compliance with Standard C++, and when entered correctly, they will not generate any
troublesome warning messages.
For the examples in this book, you will want to use your compiler’s default (or “normal”) error reporting.
However, you should examine your compiler’s documentation to see what options you have at your
disposal. Many compilers have sophisticated features that can help you spot subtle errors before they
become big problems. Understanding your compiler’s error reporting system is worth your time and
effort.
1. Where does a C++ program begin execution?
2. What is cout?
3. What does #include do?
Answer Key:
1. A C++ program begins execution with main( ).
2. cout is a predefined identifier that is linked to console output.
3. It includes the header , which supports I/O.
CRITICAL SKILL 1.5: A Second Simple Program
Perhaps no other construct is as fundamental to programming as the variable. A variable is a named
memory location that can be assigned a value. Further, the value of a variable can be changed during
the execution of a program. That is, the content of a variable is changeable, not fixed.
The following program creates a variable called length, gives it the value 7, and then displays the
message “The length is 7” on the screen.
15 C++ A Beginner’s Guide by Herbert Schildt

As mentioned earlier, the names of C++ programs are arbitrary. Thus, when you enter this program,
select a filename to your liking. For example, you could give this program the name VarDemo.cpp.
This program introduces two new concepts. First, the statement
int length; // this declares a variable
declares a variable called length of type integer. In C++, all variables must be declared before they are
used. Further, the type of values that the variable can hold must also be specified. This is called the type
of the variable. In this case, length may hold integer values. These are whole number values whose
range will be at least –32,768 through 32,767. In C++, to declare a variable to be of type integer, precede
its name with the keyword int. Later, you will see that C++ supports a wide variety of built-in variable
types. (You can create your own data types, too.)
The second new feature is found in the next line of code:
length = 7; // this assigns 7 to length
As the comment suggests, this assigns the value 7 to length. In C++, the assignment operator is the
single equal sign. It copies the value on its right side into the variable on its left. After the assignment,
the variable length will contain the number 7.
The following statement displays the value of length:
cout

on the screen. Before moving on, you might want to try giving length other values and watching the
results.
CRITICAL SKILL 1.6: Using an Operator
Like most other computer languages, C+ supports a full range of arithmetic operators that enable you to
manipulate numeric values used in a program. They include those shown here:
These operators work in C++ just like they do in algebra.
The following program uses the * operator to compute the area of a rectangle given its length and the
width.
This program declares three variables: length, width, and area. It assigns the value 7 to length and the
value 5 to width. It then computes the product and assigns that value to area. The program outputs the
following:
The area is 35
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In this program, there is actually no need for the variable area. For example, the program can be
rewritten like this:
In this version, the area is computed in the cout statement by multiplying length by width. The result is
then output to the screen.
One more point before we move on: It is possible to declare two or more variables using the same
declaration statement. Just separate their names by commas. For example, length, width, and area
could have been declared like this:
int length, width, area; // all declared using one statement
Declaring two or more variables in a single statement is very common in professionally written C++
code.
1. Must a variable be declared before it is used?
2. Show how to assign the variable min the value 0.
3. Can more than one variable be declared in a single declaration statement?
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Answer Key:
1. Yes, in C++ variables must be declared before they are used.
2. min = 0;
3. Yes, two or more variables can be declared in a single declaration statement.
CRITICAL SKILL 1.7: Reading Input from the Keyboard
The preceding examples have operated on data explicitly specified in the program. For example, the
area program just shown computes the area of a rectangle that is 7 by 5, and these dimensions are part
of the program itself. Of course, the calculation of a rectangle’s area is the same no matter what its size,
so the program would be much more useful if it would prompt the user for the dimensions of the
rectangle, allowing the user to enter them using the keyboard.
To enable the user to enter data into a program from the keyboard, you will use the >> operator. This is
the C++ input operator. To read from the keyboard, use this general form
cin >> var;
Here, cin is another predefined identifier. It stands for console input and is automatically supplied by
C++. By default, cin is linked to the keyboard, although it can be redirected to other devices. The variable
that receives input is specified by var.
Here is the area program rewritten to allow the user to enter the dimensions of the rectangle:
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Here is a sample run:
Enter the length: 8
Enter the width: 3
The area is 24
Pay special attention to these lines:
cout > length; // input the length
The cout statement prompts the user. The cin statement reads the user’s response, storing the value in
length. Thus, the value entered by the user (which must be an integer in this case) is put into the
variable that is on the right side of the >> (in this case, length). Thus, after the cin statement executes,
length will contain the rectangle’s length. (If the user enters a nonnumeric response, length will be
zero.) The statements that prompt and read the width work in the same way.
Some Output Options
So far, we have been using the simplest types of cout statements. However, cout allows much more
sophisticated output statements. Here are two useful techniques. First, you can output more than one
20 C++ A Beginner’s Guide by Herbert Schildt